1. Field of Invention
This invention relates to a process to convert natural gas into liquid fuels and chemicals. In particular, the present invention relates to a process utilizing vacuum pressure swing adsorption to produce high purity oxygen to react in an autothermal reformer to produce synthesis gas.
2. Prior Art
The Fischer-Tropsch (“FT”) Synthesis has been used to convert synthesis gas (carbon monoxide and hydrogen) into hydrocarbon products. These resulting hydrocarbon products can be useful as a synthetic crude oil or further refined into various fuels, chemicals and chemical intermediate products. The FT feedstock synthesis gas can be produced using a wide range of raw materials including, for example, coal, biomass and natural gas. The method of process design and integration is, essential to any effort to convert a raw material into useful hydrocarbon products by FT chemistry. Conversion of the raw material into synthesis gas is often the critical step as it is capital intensive.
Production of synthesis gas requires the introduction of oxygen and/or heat. The source of oxygen can be atmospheric air, enriched air, substantially pure oxygen (90%+O2), steam, or a combination of these.
When the feedstock is natural gas, historically steam has been used as the source to obtain oxygen. This process is known as steam methane reforming. An example of steam methane reforming may be seen in Tio (U.S. Pat. No. 7,550,635). Methane is partially oxidized in a non-catalyzed reaction followed by steam reforming of methane.
Prasad et al. (U.S. Pat. No. 6,695,983) discloses another example of steam methane reforming.
In recent years however, autothermal reforming has gained acceptance for syngas production. Autothermal reforming uses a small amount of steam and substantially pure oxygen to produce synthesis gas by a combination of combustion and reforming reactions. The exothermic partial oxidation reaction provides the necessary heat for endothermic steam methane reforming reactions that occur over a single catalyst bed.
The commercial development of these processes has been driven to larger scale in order to gain advantage from economy of scale. Since steam methane reforming requires a large number of small catalyst filled tubes in a large fired heater, the autothermal reforming process with a single catalyst bed has become a preferred method for many processes requiring synthesis gas. The autothermal reformer is particularly preferred for use with a FT process because it produces synthesis gas at or near the ideal 2/1 hydrogen to carbon monoxide H2/CO molar ratio.
While the technology development is being driven by economy of scale, there are commercial reasons to consider smaller scale development. Natural gas fields vary significantly in size and location, and a substantial portion of the natural gas discovered is stranded and, therefore, has no ready access to a market. This lack of market access is a result of the difficulty to transport the natural gas to a market. The smaller gas fields are disadvantaged compared to larger fields due to limited pipeline access. If the natural gas could be converted to liquid fuels or chemical intermediates it can easily be transported to a market. There are also many more small fields than large fields. Therefore, there is a need to develop a process that can convert natural gas into fuels and/or chemicals efficiently and economically at a relatively small scale.
Beer (U.S. Pat. No. 5,755,840) suggests using an oxygen-sorbent material to add oxygen to a feed stream where the feed stream can be natural gas. The combined feed stream and oxygen can be passed to a reactor for conversion to syngas. In this process, the feed gas is used to desorb the oxygen off of the sorbent. The feed gas/oxygen mixture is a combustible mixture and therefore the time it can be retained prior to reaction is very short. Also, the ability to preheat the mixture is very limited and the process may well be found to be hazardous to practice.
Accordingly, there remains a need for a safe effective method to produce syngas from natural gas at a relatively small scale.
Other processes to obtain enriched air or substantially pure oxygen have been proposed. Baksh et al. (U.S. Pat. No. 7,867,320) discloses a specific mechanical design using a vacuum pressure swing adsorption process.
There remains a need to adapt vacuum pressure swing adsorption technology to conversion of natural gas into liquid fuels and chemicals.